The present invention relates to a liquid ejection head for electrostatic ink jet, which ejects droplets by exerting electrostatic forces on a solution in which charged particles are dispersed, and a method of manufacturing the liquid ejection head.
Known examples of liquid ejection heads (hereinafter referred to as the “ejection heads”) for ink jet that perform image recording (drawing) by ejecting ink droplets include an ejection head for so-called thermal ink jet that ejects ink droplets by means of expansive forces of bubbles generated in ink through heating of the ink, and an ejection head for so-called piezoelectric-type ink jet that ejects ink droplets by giving pressures to ink using piezoelectric elements.
In the case of the thermal ink jet, however, the ink is partially heated to 300° C. or higher, so there arises a problem in that a material of the ink is limited. On the other hand, in the case of the piezoelectric-type ink jet, there occurs a problem in that a complicated structure is used and an increase in cost is inevitable.
Known as ink jet that solves the problems described above is electrostatic ink jet which uses ink containing charged colorant particles (fine particles), exerts electrostatic forces on the ink, and ejects ink droplets by means of the electrostatic forces.
An ejection head for the electrostatic ink jet includes an insulating ejection substrate, in which many through holes (ejection openings) for ejecting ink droplets are formed, and ejection electrodes that respectively correspond to the ejection openings, and ejects ink droplets by exerting electrostatic forces on ink through application of predetermined voltages to the ejection electrodes. More specifically, with the construction, the ejection head ejects the ink droplets by controlling on/off of the voltage application to the ejection electrodes (modulation-driving the ejection electrodes) in accordance with image data, thereby recording an image corresponding to the image data onto a recording medium.
An example of such an ejection head for the electrostatic ink jet is disclosed in JP 10-230608 A as an ejection head 200. As conceptually shown in FIG. 11, the ejection head 200 includes a support substrate 202, an ink guide 204, an ejection substrate 206, an ejection electrode 208, a bias voltage supply 212, and a drive voltage supply 214.
In the ejection head 200, the support substrate 202 and the ejection substrate 206 are each an insulating substrate and are arranged to be spaced apart from each other by a predetermined distance.
Many through holes (substrate through holes) that each serve as an ejection opening 218 for an ink droplet are formed in the ejection substrate 206, and a gap between the support substrate 202 and the ejection substrate 206 is set as an ink flow path 216 that supplies ink Q to the ejection opening 218. In addition, the ring-shaped ejection electrode 208 is provided to an upper surface (ink-droplet-R-ejection-side surface) of the ejection substrate 206 to surround the ejection opening 218. The bias voltage supply 212 and the drive voltage supply 214 that is a pulse voltage supply are connected to the ejection electrode 208, which is grounded through the voltage supplies 212 and 214.
On the other hand, the ink guide 204 is provided to the support substrate 202, corresponding to each ejection opening 218, and protrudes from the ejection substrate 206 while passing through the ejection opening 218. Also, an ink guide groove 220 for supplying the ink Q to a tip end portion 204a of the ink guide 204 is formed by cutting out the tip end portion 204a by a predetermined width.
In an (ink jet) recording apparatus disclosed in JP 10-230608 A using the ejection head 200 described above, at the time of image recording, a recording medium P is supported by a counter electrode 210.
The counter electrode 210 functions not only as a counter electrode for the ejection electrode 208 but also as a platen supporting the recording medium P at the time of the image recording and is arranged to face the upper surface of the ejection substrate 206 and to be spaced apart from the tip end portion 204a of the ink guide 204 by a predetermined distance.
In the ejection head 200, at the time of the image recording, an ink circulation mechanism (not shown) causes the ink Q containing the charged colorant particles to flow in the ink flow path 216 in a direction, for instance, from the right side to the left side in the drawing. Note that the colorant particles of the ink Q are charged to the same polarity as the voltage applied to the ejection electrode 208.
The recording medium P is supported by the counter electrode 210 and faces the ejection substrate 206.
Further, a DC voltage of, for example, 1.5 kV is constantly applied from the bias voltage supply 212 to the ejection electrode 208 as a bias voltage.
As a result of the ink Q circulation and the bias voltage application, by the action of surface tension of the ink Q, a capillary phenomenon, an electrostatic force due to the bias voltage, and the like, the ink Q is supplied from the ink guide groove 220 to the tip end portion 204a of the ink guide 204, a meniscus of the ink Q is formed at the ejection opening 218, the colorant particles move to the vicinity of the ejection opening 218 (migration due to an electrostatic force), and the ink Q is concentrated in the ejection opening 218 and the tip end portion 204a. 
In this state, when the drive voltage supply 214 applies a pulse-shaped drive voltage of, for example, 500 V corresponding to image data (drive signal) to the ejection electrode 208, the drive voltage is superimposed on the bias voltage and the supply and concentration of the ink Q to and in the tip end portion 204a are promoted. When a movement force of the ink Q and the colorant particles to the tip end portion 204a and an attraction force from the counter electrode 14 exceed the surface tension of the ink Q, a droplet (ink droplet R) of the ink Q, in which the colorant particles are concentrated, is ejected.
The ejected ink droplet R flies due to momentum at the time of the ejection and the attraction force by the counter electrode 210, impinges on the recording medium P, and forms an image.
In addition, JP 08-149253 A discloses an electrostatic ink jet recording apparatus which includes an electrode array formed on a surface of a substrate, a supply device that supplies ink onto the electrode array, and a voltage application device that applies drive voltages to the electrode array. Further, JP 09-309208 A discloses an electrostatic ink jet recording apparatus which includes an ink supply path having many openings formed to a surface of an insulating base material and serving as nozzles, electrodes formed on the surface of the base material to surround the openings, and a supply device that supplies ink to the openings from the inside of the base material through the ink supply path.
In recent years, an increase in recording density for supporting a high resolution and an increase in speed are demanded of even such an electrostatic ink jet head (electrostatic ink jet recording apparatus).
In order to achieve the increase in recording density, it is required to form the ink ejection portions, that is, the ejection openings and the ejection electrodes (as well as the ink guides in some cases) on the substrate at a high density (it is required to increase a packaging density). In addition, two-dimensional arrangement of the ejection portions is also extremely effective for the increase in recording density and the increase in speed.
As is apparent also from the construction in each patent document described above, however, when the density of the ejection portions is increased, wiring for applying drive voltages to the respective ejection electrodes at the ejection substrate becomes complicated and increases in density and multilayering of the wiring is also required in some cases. In addition, when the ejection portions are arranged in a two-dimensional manner, the multilayering of the wiring becomes indispensable to some extent in terms of the construction.
As a result, the electrostatic ink jet ejection head has a problem in that as its recording density is increased, its structure becomes complicated. In addition, when the multilayering is achieved while maintaining ejection performance, the thickness of a wiring substrate is limited for stabilized ink supply to the ejection portions and maintenance of an inter-counter-electrode distance. Therefore, for the multilayering, it is required to reduce a distance between wires on a wiring side or reduce the thickness of an insulation layer. However, this results in a problem in that a withstand voltage is reduced.
In addition, when the ejection portions are arranged at a high density or in a two-dimensional manner, as a matter of course, distances between adjacent ejection portions are reduced, so electric field interferences occur between the adjacent ejection portions. As a result, there also occurs a problem in that, for instance, ejection becomes unstable and ejection at high speed (high recording (droplet ejection) frequency) becomes impossible.